1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor comprising internal electrodes composed of a silver-palladium alloy and a process for the preparation thereof. More particularly, the present invention relates to an improvement of the multilayer ceramic capacitor in which the equivalent series resistance is reduced and the capacitance is increased.
2. Description of the Prior Art
A commercially available mulitlayer ceramic capacitor has a structure in which a plurality of thin ceramic dielectric layers having internal electrodes formed on the surfaces thereof are laminated and integerally sintered and the internal electrodes are alternately connected in parallel to a pair of electrodes for the external connection formed on the side faces of the sintered assembly.
Recently, in some of such ceramic capacitors, a silver-palladium alloy (Ag-Pd) is used instead of palladium (Pd), which is a noble metal customarily used for internal electrodes, so as to reduce the cost of the internal electrode material.
However, if the content of silver (Ag) is low, the specific resistance of the silver-palladium alloy (Ag-Pd) is high and since silver (Ag) is evaporated during the sintering, the absolute amount of the metal forming the electrode is reduced and a net-like electrode film is formed to increase the sheet resistance to increase the equivalent series resistance (ESR) of the capacitor. Therefore, the capacitor cannot be used for a high-frequency circuit.
It may be considered that the specific resistance will be reduced by increasing the content of silver (Ag) in the silver-palladium alloy (Ag-Pd). However, if the content of silver (Ag) is increased, the melting point of the metal is lowered, and a stable mulilayer ceramic capacitor cannot be obtained. Moreover, for preventing formation of a short circuit, which is due to migration of atoms between the electrodes by an electric field, the increase of the ratio of silver (Ag) is limited. For compensation of silver (Ag) evaporated during the sintering, the amount of silver (Ag) coated and applied to the surface of a green sheet of a dielectric material may be increased. However, this results in structural defects of the multilayer ceramic capacitor (cracking or delamination) and the ceramic capacitor cannot be put into practical use.